Contamination detection in fuel system

A method for an engine, comprising:

• storing a water-based urea solution in a urea reservoir and delivering the urea solution to an SCR catalyst;
• storing fuel in a fuel tank and delivering the fuel to the engine for combustion;
• separating water-based fluid from fuel in the fuel tank via a separator into a separation reservoir;
• and
• indicating mis-filling of the urea solution into the fuel tank in response to water content of the separated fluid and a rate of change of a fluid level of the separated fluid being greater than a threshold amount.

A fuel system, comprising:

• a fuel tank for fuel;
• a fuel tank fluid level sensor coupled to the fuel tank;
• a reductant tank for water-based reductant;
• a water-based fluid separator coupled to the fuel tank;
• a reservoir coupled to the water-based fluid separator receiving separated water-based fluid from the fuel tank;
• a water-in-fuel sensor coupled to the reservoir;
• and
• a control system comprising a computer readable storage medium, the medium comprising instructions for:
• in response to a fuel refill event with no reductant refill event, indicating a first type of fuel contamination via the water-in-fuel sensor if an amount of separated water-based fluid exceeds a first threshold value and a rate of change of a fluid level of the separated water-based fluid exceeds a first threshold rate;
• and
• in response to the fuel refill event with a reductant refill event, indicating a second type of less-intensive fuel contamination via the water-in-fuel sensor if the amount of separated water-based fluid exceeds a second, lower, threshold value but not the first threshold value, and the rate of change of the fluid level of the separated water-based fluid exceeds a second, lower, threshold rate, but does not exceed the first threshold rate.